Fruit flies turn on autopilot

High-speed video shows how bugs change course in a flash

View a video of a fly ‘swimming’ through the air.

High-speed cameras plus computer models show that fruit flies “swim” through the air like rowboats. Attila Bergou/Physical Review Letters 2010

Fruit flies turn in midair with a shrug of their shoulders.

A new analysis shows that the flies’ aerial gymnastics are driven by wing joints that act like wind-up toys, letting the bugs whirl around almost automatically.

Insights from the study, which was published online April 5 in Physical Review Letters, could someday help build better flying robots.

Fruit flies beat their wings about once every four milliseconds — faster than their neurons can fire — and can turn 120 degrees in 18 wing beats. This made study coauthor Itai Cohen of Cornell University wonder, “How much of the wing motion is being controlled by the insect, and how much is going along for the ride, being controlled by aerodynamics?”

To investigate, Cohen and his colleagues set up three high-speed cameras trained at the center of a box holding about ten flies. A fly crossing the center of the box triggered the cameras to start rolling at 8,000 frames per second. At the same time, a disk of LED lights projected a rotating striped pattern on the inside of the box to trick the flies into making a U-turn.

“The flies see this, and it makes them dizzy,” says study coauthor Attila Bergou of Brown University in Providence, R.I., who helped perform the experiments as a graduate student at Cornell. “It generates very reliable and repeatable turns in these flies.”

The physicists analyzed the videos to extract detailed information on the wings’ positions with respect to the body.

“I was surprised that they were able to get it to work as well as they did,” comments Ty Hedrick of the University of North Carolina in Chapel Hill. “Getting the uncertainty of these measurements low enough that you can see what you need to see is difficult.”

The team found that when the fly turns, one wing tilts more than the other, similar to the way a rower pulls one oar harder than the other to make a boat turn. Thanks to aerodynamics, a wing-tilt difference of just 9 degrees is enough to send a fly off in another direction.

“Essentially these insects are swimming through the air, using drag forces to row themselves in whichever direction they want,” Cohen says.

Further analysis using computer models of the fly and aerodynamic simulations showed that the fly’s wing joint acts like a torsional spring, the kind found in wind-up toys or old clocks. To change its wing tilt and set up a turn, all the fly has to do is twitch the muscle that controls the spring.

“The insects don’t have to do any thinking whatsoever,” Cohen says. “They have a natural system that provides just the right amount of torque to the wing.”

The physicists are planning comparative studies in other flying insects, like bees and dragonflies. Cohen hopes the findings could help design more maneuverable flying robots that take advantage of insect aerodynamics.

“Really the idea is, how do we start to build more efficient and smaller robots that take advantage of aerodynamics to do the things they do, rather than brute force the way we usually do these things?” Cohen says. “We’re in the dark ages as far as building anything like that. We’re nowhere in the ballpark.”

Caption: High-speed cameras plus computer models show that fruit flies “swim” through the air like rowboats.

Credit: Attila Bergou/Physical Review Letters 2010

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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